Opponent in a state of stagnation (10)

I believe the answer is:

antagonist

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'opponent in a state' is the definition.
'antagonist' can be an answer for 'opponent' (synonyms). I am not sure about the 'in a state' bit.

'of stagnation' is the wordplay.
'of' indicates anagramming the letters (the answer comes from or 'of' the letters).
'stagnation' anagrammed gives 'ANTAGONIST'.

Can you help me to learn more?

(Other definitions for antagonist that I've seen before include "Adversary" , "Opponent" , "Hostile person" , "rival" , "5 down (anag)" .)

I've seen this clue in The Irish News.

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